This invention relates generally to methods and apparata for manufacturing thermoplastic structures and more particularly concerns the use of ultrasonic vibrations to induce flow of resin in resin impregnated fiber substructures at temperatures below the melt temperature of the resin.
Pultrusion has proven to be a very effective process in the manufacture of thermoset structures. Typically, the process employs a thermoset resin in its liquid state in a bath in which the reinforcing fibers are coated and impregnated. The impregnated fibers are pulled through dies to shape the structure and heated to cure the thermoset resin to a hard, solid matrix.
Attempts to apply the pultrusion process to thermoplastic resins have met with considerable difficulty. When heated above the melting point, the high viscosity of a thermoplastic resin makes complete impregnation of the reinforcing fibers very difficult. Inconsistent impregnation means inconsistent finished products. Furthermore, this high viscosity also results in exertion of very high pull forces on the resin coated fibers. The high pull forces in turn cause a relatively high frequency of breakage of the fiber chain during the pultrusion process. This translates into costly work interruptions. These problems are exacerbated when the pultrusion process is used in the production of profiles such as T sections, channels, angles, tubular members or other more complex cross sectional shapes.
In order to mitigate these difficulties, the present thermoplastic pultrusion processes compromise efficiency. Most notably, the pull rate of present systems is generally limited to two to three inches of product profile per minute. Maximum pull rates are achieved only in specific applications depending on the choice of resin and profile configuration and still approximate only two feet per minute. This is far below the pull rates achieved in the pultrusion of thermoset profiles.
This comparative inefficiency of the thermoplastic pultrusion process is unacceptable because thermoplastic structures offer tremendous advantages over thermoset structures. For example, thermoplastic structures, unlike thermosets, require no curing, thus simplifying processing and affording essentially infinite storage life.
Among the present solutions offered for the difficulty is the use of extended melt time to permit a more thorough impregnation of the fibers. While this affords a more consistent finished product, it obviously decreases the production rate of the system. Another alternative is the use of polymers having lower melt viscosities. While this is appropriate for many applications, it is an unfortunate concession to undesirable limitations. Other solutions utilize ultrasonic vibration at high power input to increase the melt rate of the resin, to increase the temperature of the melted resin or to overcome the frictional forces impeding the pull of a profile which has been heated above melt temperature. These solutions are bandaids which cover the problems of high viscosity of melted thermoplastic resins rather than cure them.
It is therefore an object of this invention to increase the efficiency of the pultrusion process as applied to thermoplastic resins. It is a further object of this invention to reduce the pull forces on thermoplastic profiles which cause their breakage and consequential interruptions of the manufacturing process. It is, therefore, an object of this invention to permit the pultrusion of thermoplastic profiles at temperatures below the melt temperature of the resin. Some collateral objects of this invention are to enable use of a wider selection of thermoplastic resins, to minimize the risk of degradation of thermoplastic polymers including those of higher molecular weights, to provide a more uniform thermoplastic profile having minimal porosity and voids in its final structure, to reduce the warpage that ordinarily occurs during cooling of a molten thermoplastic polymer, to reduce the sensitivity of the system to variations in temperature which would result in defects in the finished product and to reduce wear and damage to the pultrusion process equipment.